Method for preparing 1,3-butadiene from n-butenes by oxidative dehydrogenation

ABSTRACT

The invention relates to a method for producing butadiene from n-butenes having the steps:
     A) providing a feed gas stream a comprising n-butenes;   B) feeding the feed gas stream a comprising the n-butenes and an oxygen-comprising gas into at least one oxidative dehydrogenation zone and oxidatively dehydrogenating n-butenes to butadiene, wherein a product gas stream b comprising butadiene, unreacted n-butenes, steam, oxygen, low-boiling hydrocarbons, high-boiling minor components, possibly carbon oxides and possibly inert gases is obtained;   Ca) cooling the product gas stream b by contacting it with a refrigerant and condensing at least a part of the high-boiling minor components;   Cb) compressing the remaining product gas stream b in at least one compression stage, wherein at least one aqueous condensate stream c 1  and a gas stream c 2  comprising butadiene, n-butenes, steam, oxygen, low-boiling hydrocarbons, possibly carbon oxides and possibly inert gases are obtained;   Da) separating off non-condensable and low-boiling gas components comprising oxygen, low-boiling hydrocarbons, possibly carbon oxides and possibly inert gases as gas stream d 2  from the gas stream c 2  by absorbing the C 4  hydrocarbon-comprising butadiene and n-butenes in an absorbent, wherein an absorbent stream loaded with C 4  hydrocarbons and the gas stream d 2  are obtained, and   Db) subsequent desorption of the C 4  hydrocarbons from the loaded absorbent stream in a desorption column, wherein a C 4  product gas stream d 1  is obtained,
 
wherein a polymerization inhibitor is added in step Db) at the column head of the desorption column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application (under 35 U.S.C. § 371)of PCT/EP2017/050438, filed Jan. 11, 2017, which claims benefit ofEuropean Application No. 16151045.8, filed Jan. 13, 2016, both of whichare incorporated herein by reference in their entirety.

The invention relates to a method for producing 1,3-butadiene fromn-butenes by oxidative dehydrogenation (ODH), in which a polymerizationinhibitor is added in the work-up part.

Butadiene is an important basic chemical and is used, for example, forproducing synthetic rubbers (butadiene homopolymers, styrene-butadienerubber or nitrile rubber) or for producing thermoplastic terpolymers(acrylonitrile-butadiene-styrene copolymers). Butadiene is additionallyreacted to form sulfolane, chloroprene and 1,4-hexamethylenediamine (via1,4-dichlorobutene and adipodinitrile). By dimerizing butadiene, inaddition, vinylcyclohexene can be generated, which can be dehydrogenatedto form styrene.

Butadiene can be produced by the thermal cracking (steam cracking) ofsaturated hydrocarbons, customarily starting from naphtha as rawmaterial. In the steam cracking, of naphtha, a hydrocarbon mixture ofmethane, ethane, ethene, acetylene, propane, propene, propyne, allene,butanes, butenes, butadiene, butynes, methylallene, C₅— and higherhydrocarbons is produced.

Butadiene can also be obtained by oxidative dehydrogenation of n-butenes(1-butene and/or 2-butene). The starting mixture used for the oxidativedehydrogenation (oxy dehydrogenation, ODH) of n-butenes to formbutadiene can be any desired mixture containing n-butenes. For example,a fraction can be used that, as main component, comprises n-butenes(1-butene and/or 2-butene) and was obtained from the C₄ fraction of anaphtha cracker by separating off butadiene and isobutene. In addition,gas mixtures can be used as starting gas, which mixtures comprise1-butene, cis-2-butene, trans-2-butene or mixtures thereof, and wereobtained by dimerizing ethylene. In addition, as starting gas, gasmixtures comprising n-butenes can be used, which mixtures were obtainedby fluid catalytic cracking (FCC).

Methods for the oxidative dehydrogenation of butenes to butadiene areknown in principle. Such methods frequently comprise the followingsteps:

-   A) providing a feed gas stream a comprising n-butenes;-   B) feeding the feed gas stream a comprising n-butenes and an    oxygen-comprising gas n-butenes to butadiene, wherein a product gas    stream b comprising butadiene, unreacted n-butenes, steam, oxygen,    low-boiling hydrocarbons, high-boiling minor components, possibly    carbon oxides and possibly inert gases is obtained;-   Ca) cooling the product gas stream b by contacting it with a    refrigerant and condensing at least a part of the high-boiling minor    components;-   Cb) compressing the remaining product gas stream b in at least one    compression stage, wherein at least one aqueous condensate stream c1    and a gas stream c2 comprising butadiene, n-butenes, steam, oxygen,    low-boiling hydrocarbons, possibly carbon oxides and possibly inert    gases are obtained;-   Da) separating off non-condensable and low-boiling gas components    comprising oxygen, low-boiling hydrocarbons, possibly carbon oxides    and possibly inert gases as gas stream d2 from the gas stream c2 by    absorbing the C₄ hydrocarbon-comprising butadiene and n-butenes in    an absorbent, wherein an absorbent stream loaded with C₄    hydrocarbons and the gas stream d2 are obtained, and-   Db) subsequent desorption of the C₄ hydrocarbons from the loaded    absorbent stream in a desorption column, wherein a C₄ product gas    stream d1 is obtained.

US 2012/0130137A1, for example, describes a method for the oxidativedehydrogenation of butenes to butadiene, using catalysts which compriseoxides of molybdenum, bismuth and generally further metals.

In paragraph [0122], the problems of byproducts are also referred to. Inparticular phthalic anhydride, anthraquinone and fluorenone arementioned, which are typically present in the product gas atconcentrations from 0.001 to 0.10% by volume. In US 2012/0130137A1,paragraph [0124] to [0126], it is recommended to cool the hot reactordischarge gases directly to firstly 5 to 100° C. by contacting them witha coolant liquid (quench tower). Water or aqueous alkali solutions arementioned as coolant liquids. The problems of blockages in the quenchdue to high-boilers from the product gas or due to polymerizationproducts of high-boiling byproducts from the product gas are explicitlymentioned as coolant liquids, for which reason it is said to beadvantageous that high-boiling byproducts are discharged as little aspossible from the reaction part into the cooling part (quench).Separating off isobutene from the decomposition product thereof,methacrolein, from acetaldehyde or from acrolein is not mentioned.

In JP 2011-001341A, a two-stage cooling is described for a method forthe oxidative dehydrogenation of alkenes to conjugated alkadienes. Inthis case, the product discharge gas of the oxidative dehydrogenation isfirst adjusted to a temperature between 300 and 221° C. and then furthercooled to a temperature between 99 and 21° C. In paragraphs [0066] f.,it is described that, to set the temperature between 300 and 221° C.,heat exchangers are preferably used, wherein, however, a part of thehigh-boilers could also precipitate out of the product gas in said heatexchangers. In JP2011-001341A, therefore, an occasional washing out ofdeposits from the heat exchangers using organic or aqueous solvents isdescribed. As solvents, for example, aromatic hydrocarbons such astoluene or xylene, or an alkaline aqueous solvent such as, for example,the aqueous solution of sodium hydroxide are described. In order toavoid too-frequent shutting down of the method for cleaning the heatexchangers, in JP 2011-001341A, a structure having two parallel-arrangedheat exchangers is described, which are each operated or purgedalternately (what is termed A/B procedure). A separation from isobuteneor from the decomposition product thereof, methacrolein, of fromacetaldehyde and acrolein is not mentioned.

JP 2011-132218 restricts the isobutene content in the feed, since it isknown that isobutene forms oxygenates. Separating off the oxygenates,however, is not described.

JP 2012240963 describes a method for butadiene production in which theC₄ hydrocarbon-comprising gas stream is contacted with an absorbent b inan absorbent stage b′, in order to absorb the C₄ components.

JP 2010-090083 limits the amount of aldehydes and also discloses intable 1 the formation of methacrolein, but makes no proposal onseparating it off.

Isobutene is present in virtually all C₄ hydrocarbon streams that can beused for the ODH process. In particular, C₄ hydrocarbon streams from FCcrackers comprise isobutene in amounts of up to 15% by volume. Theisobutene entering the ODH reactor is, depending on the catalyst usedand the reaction conditions, converted to methacrolein by approximately50%. Said methacrolein accumulates in the circuit stream of theabsorption/desorption part of the C₄ hydrocarbon removal and can causeside reactions such as oligomerizations and polymerizations, deposits onthe column internals, and in particular on evaporators and condensers,and also an impairment of the separation efficiency.

It has been found that the minor components present in the product gasstream of the ODH reactor initiate and promote polymer formation inregions of the downstream work-up stages in which high concentrations ofC₄ hydrocarbons are present.

In JP 2011-006381 A, the risk of peroxide formation in the work-up partof a method for producing conjugated alkadienes is addressed. To solvethis problem, the addition of polymerization inhibitors to theabsorption solutions of the C₄ hydrocarbon removal and setting a maximumperoxide content of 100 ppm by weight by heating absorption solutions isdescribed.

The object of the invention is to provide an improved method forproducing butadiene by oxidative dehydrogenation of n-butenes andsubsequent work-up of the product gas stream comprising C₄ hydrocarbonsand minor components, which method provides a remedy to thedisadvantages described above. In particular, a method is to be providedin which polymer formation is minimized during the removal of the C₄hydrocarbons from the product gas stream of the ODH reactor in thework-up part of the method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the method according to the invention.

The object is achieved by a method for producing butadiene fromn-butenes having the steps:

-   A) providing a feed gas stream a comprising n-butenes;-   B) feeding the feed gas stream a comprising n-butenes and an    oxygen-comprising gas into at least one oxidative dehydrogenation    zone and oxidatively dehydrogenating n-butenes to butadiene, wherein    a product gas stream b comprising butadiene, unreacted n-butenes,    steam, oxygen, low-boiling hydrocarbons, high-boiling minor    components, possibly carbon oxides and possibly inert gases is    obtained;-   Ca) cooling the product gas stream b by contacting it with a    refrigerant and condensing at least a part of the high-boiling minor    components;-   Cb) compressing the remaining product gas stream b in at least one    compression stage, wherein at least one aqueous condensate stream c1    and a gas stream c2 comprising butadiene, n-butenes, steam, oxygen,    low-boiling hydrocarbons, possibly carbon oxides and possibly inert    gases are obtained;-   Da) separating off non-condensable and low-boiling gas components    comprising oxygen, low-boiling hydrocarbons, possibly carbon oxides    and possibly inert gases as gas stream d2 from the gas stream c2 by    absorbing the C₄ hydrocarbon-comprising butadiene and n-butenes in    an absorbent, wherein an absorbent stream loaded with C₄    hydrocarbons and the gas stream d2 are obtained, and-   Db) subsequent desorption of the C₄ hydrocarbons from the loaded    absorbent stream in a desorption column, wherein a C₄ product gas    stream d1 is obtained,    wherein a polymerization inhibitor is added in step Db) at the    column head of the desorption column.

Generally, a top condenser is situated at the column head of thedesorption column. Preferably, the polymerization inhibitor is added inthe region of the top condenser.

It has been found that the polymer formation in the desorption column,in the top condenser of the desorption column, in the top condensercircuit and also in downstream evaporators can be minimized by theaddition of polymerization inhibitors.

Preferably, the polymerization inhibitor is added in amounts such thatthe concentration of the polymerization inhibitor in the liquidcondensate stream obtained at the top condenser is from 10 to 500 ppm.

Preferably, the steps E) and F) are further carried out:

-   E) separating the C₄ product stream d1 by extractive distillation    using a solvent selective for butadiene into a material stream e1    comprising butadiene and the selective solvent, and a material    stream e2 comprising n-butenes;-   F) distilling the material stream e1 comprising butadiene and the    selective solvent into a material stream f1 substantially comprising    the selective solvent, and a material stream f2 comprising    butadiene.

Generally, an aqueous refrigerant or an organic solvent is used in thecooling stage Ca).

Preferably, an organic solvent is used in the cooling stage Ca). Thesegenerally have a very much higher solvent capacity for the high-boilingbyproducts that can lead to deposits and blockages in the plant partsdownstream of the ODH reactor than water or alkaline-aqueous solutions.Preferred organic solvents used as refrigerants are aromatichydrocarbons, for example toluene, o-xylene, m-xylene, p-xylene,diethylbenzenes, triethylbenzenes, diisopropylbenzenes,triisopropylbenzenes and mesitylene (TMB) or mixtures thereof.Particular preference is given to mesitylene.

Embodiments hereinafter are preferred or particularly preferred variantsof the inventive method.

Stage Ca) is carried out in a multistage manner in stages Ca1) to Can),preferably in a two-stage manner in two stages Ca1) and Ca2). In thiscase, particularly preferably, at least a part of the solvent, afterpassage through the second stage Ca2), is fed as cooling agent to thefirst stage Ca1).

Stage Cb) generally comprises at least one compression stage Cba) and atleast one coolant stage Cbb). Preference is given to at least onecooling stage Cbb), in which the gas that is compressed in thecompression stage Cba) is contacted with a cooling agent. Particularlypreferably, the cooling agent of the cooling stage Cbb) comprises thesame organic solvent that is used as cooling agent in stage Ca). In aparticularly preferred variant, at least a part of this cooling agent,after it passes through the at least one cooling stage Cbb, is fed ascooling agent to stage Ca).

Preferably, the stage Cb) comprises a plurality of compression stagesCba1) to Cban) and cooling stages Cbb1) to Cbbn), for example fourcompression stages Cba1) to Cba4) and four cooling stages Cbb1) toCbb4).

Preferably, step D) comprises steps Da1), Da2) and Db):

-   Da1) absorption of the C₄ hydrocarbons comprising butadiene and    n-butenes in a high-boiling absorbent, wherein an absorbent stream    loaded with C₄ hydrocarbons and the gas stream d2 are obtained,-   Da2) removal of oxygen from the absorbent stream of step Da) that is    loaded with C₄ hydrocarbons by stripping with a non-condensable gas    stream, and-   Db) desorption of the C₄ hydrocarbons from the loaded absorbent    stream, wherein a C₄— product gas stream d1 is obtained which    substantially comprises C₄ hydrocarbons and comprises less than 100    ppm of oxygen.

Preferably, the high-boiling absorbent used in step Da) is an aromatichydrocarbon solvent, particularly preferably it is the aromatichydrocarbon solvent used in step Ca), in particular mesitylene.Diethylbenzenes, triethylbenzenes, diisopropylbenzenes andtriisopropylbenzenes can also be used.

In an embodiment of the invention, the gas stream d2 present in step Da)is up to at least 30%, preferably up to at least 40%, recirculated tostep B). This can be expedient if only a small purge stream has to beejected from the gas stream d2.

An embodiment of the method according to the invention is shown in FIG.1 and is described in detail hereinafter.

As feed gas stream, gas mixtures comprising n-butenes (1-butene and/orcis-/trans-2-butene), and isobutene are used. Such a gas mixture can beobtained, for example, by non-oxidative dehydrogenation of n-butane. Afraction can also be used that comprises, as main component, n-butenes(1-butene and cis-/trans-2-butene) and was obtained from the C₄ fractionof the naphtha-cracking by separating off butadiene and isobutene. Inaddition, gas mixtures can be used as starting gas stream that comprise1-butene, cis-2-butene, trans-2-butene or mixtures thereof, and whichwere obtained by dimerizing ethylene. In addition, as input gas stream,gas mixtures containing n-butenes can be used which were obtained byfluid catalytic cracking (FCC).

In an embodiment of the method according to the invention, the startinggas mixture comprising n-butenes is obtained by non-oxidativedehydrogenation of n-butane. By the coupling of a non-oxidativecatalytic dehydrogenation to the oxidative dehydrogenation of then-butenes formed, a high yield of butadiene, based on the n-butane used,can be obtained. In the non-oxidative catalytic n-butanedehydrogenation, a gas mixture is obtained which, in addition tobutadiene, contains 1-butene, 2-butene and unreacted n-butane minorcomponents. Customary minor components are hydrogen, steam, nitrogen, COand CO₂, methane, ethane, ethene, propane and propene. The compositionof the gas mixture leaving the first dehydrogenation zone can varygreatly, depending on the procedure of the dehydrogenation. Forinstance, when the dehydrogenation is carried out with feed-in of oxygenand additional hydrogen, the product gas mixture can have acomparatively high content of steam and carbon oxides. In the case ofprocedures without feed-in of oxygen, the product gas mixture of thenon-oxidative dehydrogenation has a comparatively high content ofhydrogen.

In step B), the feed gas stream comprising the n-butenes and anoxygen-comprising gas are fed into at least one dehydrogenation zone(the ODH reactor A) and the butenes present in the gas mixture aredehydrogenated to butadiene oxidatively in the presence of an oxydehydrogenation catalyst.

In an embodiment, it is preferred to use an oxygen-comprising gas thatcomprises more than 10% by volume, preferably more than 15% by volume,and particularly preferably more than 20% by volume, of molecularoxygen. In an embodiment, air is used as oxygen-comprising gas. Theupper limit for the content of molecular oxygen in the oxygen-comprisinggas is then generally 50% by volume or less, preferably 30% by volume orless, and still more preferably 25% by volume or less. Furthermore, anydesired inert gases can be present in the molecular oxygen-comprisinggas. As possible inert gases, nitrogen, argon, neon, helium, CO, CO₂ andwater can be cited. The amount of inert gases in the oxygen-comprisinggas is, for nitrogen, generally 90% by volume or less, preferably 85% byvolume or less, and still more preferably 80% by volume or less. In thecase of components other than nitrogen in the oxygen-comprising gas, theamount is generally 10% by volume or less, preferably 1% by volume orless.

Catalysts suitable for the oxy dehydrogenation are generally based on aMo—Bi—O-comprising multimetal oxide system which generally additionallycomprises iron. Generally, the catalyst system also comprises furtheradditional components such as, for example, potassium, cesium,magnesium, zirconium, chromium, nickel, cobalt, cadmium, tin, lead,germanium, lanthanum, manganese, tungsten, phosphorus, cerium, aluminumor silicon. Iron-containing ferrites have also been proposed ascatalysts.

In a preferred embodiment, the multimetal oxide comprises cobalt and/ornickel. In a further preferred embodiment, the multimetal oxidecomprises chromium. In a further preferred embodiment, the multimetaloxide comprises manganese.

Examples of Mo—Bi—Fe—O-comprising multimetal oxides are Mo—Bi—Fe—Cr—O orMo—Bi—Fe—Zr—O-comprising multimetal oxides. Preferred systems aredescribed, for example, in U.S. Pat. No. 4,547,615(Mo₁₂BiFe_(0.1)Ni₈ZrCr₃K_(0.2)O_(x) andMo₁₂BiFe_(0.1)Ni₈AlCr₃K_(0.2)O_(x)), U.S. Pat. No. 4,424,141(Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)P_(0.5)K_(0.1)O_(x)+SiO₂), DE-A 25 30 959(Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)Cr_(0.5)K_(0.1)O_(x),Mo_(13.75)BiFe₃Co_(4.5)Ni_(2.5)Ge_(0.5)K_(0.8)O_(x),Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)Mn_(0.5)K_(0.1)O_(x) andMo₁₂BiFe₃Co_(4.5)Ni_(2.5)La_(0.5)K_(0.1)O_(x)), U.S. Pat. No. 3,911,039(Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)Sn_(0.5)K_(0.1)O_(x)), DE-A 25 30 959 and DE-A24 47 825 (Mo₁₂BiFe₃Co_(4.5)Ni_(2.5)W_(0.5)K_(0.1)O_(x)).

Suitable multimetal oxides and production thereof are additionallydescribed in U.S. Pat. No. 4,423,281 (Mo₁₂BiNi₈Pb_(0.5)Cr₃K_(0.2)O_(x)and Mo₁₂Bi_(b)Ni₇Al₃Cr_(0.5)K_(0.5)O_(x)), U.S. Pat. No. 4,336,409(Mo₁₂BiNi₆Cd₂Cr₃P_(0.5)O_(x)), DE-A 26 00 128(Mo₁₂BiNi_(0.5)Cr₃P_(0.5)Mg_(7.5)K_(0.1)O_(x)+SiO₂) and DE-A 24 40 329(Mo₁₂BiCo_(4.5)Ni_(2.5)Cr₃P_(0.5)K_(0.1)O_(x)).

Particularly preferred catalytically active multimetal oxides comprisingmolybdenum and at least one further metal have the general formula (Ia):Mo₁₂Bi_(a)Fe_(b)CO_(c)Ni_(d)Cr_(e)X¹ _(f)X² _(g)O_(y)  (Ia),where

-   X¹=Si, Mn and/or Al,-   X²=Li, Na, K, Cs and/or Rb,-   0.2≤a≤1,-   0.5≤b≤10,-   0≤c≤10,-   0≤d≤10,-   2≤c+d≤10,-   0≤e≤2,-   0≤f≤10,-   0≤g≤0.5,-   y= a number which is determined under the precondition of charge    neutrality by the valency and frequency of the element different    from oxygen in (Ia).

Preference is given to catalysts whose catalytically active oxide massof the two metals Co and Ni has only Co (d=0). Preferably X¹ is Siand/or Mn and X² is preferably K, Na and/or Cs, particularly preferablyX²=K. Particular preference is given to a substantially Cr(VI)-freecatalyst.

To carry out the oxidative dehydrogenation at a high overall conversionrate of n-butenes, a gas mixture is preferred that has a molaroxygen:n-butene ratio of at least 0.5. Preferably, an oxygen:n-buteneratio from 0.55 to 10 is employed. To establish this value, the startinggas can be mixed with oxygen or an oxygen-comprising gas and optionallyadditional inert gas, methane or steam. The resultant oxygen-comprisinggas mixture is then fed to the oxy dehydrogenation.

The reaction temperature of the oxy dehydrogenation is generallycontrolled by a heat-exchange medium which is located around thereaction tubes. As such liquid heat-exchange media, e.g. melts of saltsor salt mixtures such as potassium nitrate, potassium nitrite, sodiumnitrite and/or sodium nitrate and also melts of metals such as sodium,mercury and alloys of various metals come into consideration. However,ionic liquids or heat carrier oils are also usable. The temperature ofthe heat-transfer medium is between 220 and 490° C., and preferablybetween 300 and 450° C., and particularly preferably between 350 and420° C.

On account of the exothermy of the reactions that proceed, thetemperature can be higher in certain sections of the reactor interiorduring the reaction than those of the heat-exchange medium, and what istermed a hotspot forms. The position and height of the hotspot isestablished by the reaction conditions, but they can also be regulatedby the dilution ratio of the catalyst layer or the throughput of mixedgas. The difference between hotspot temperature and the temperature ofthe heat-exchange medium is generally between 1 and 150° C., preferablybetween 10 and 100° C., and particularly preferably between 20 and 80°C. The temperature at the end of the catalyst bed is generally between 0and 100° C., preferably between 0.1 and 50° C., particularly preferablybetween 1 and 25° C. above the temperature of the heat-exchange medium.

The oxy dehydrogenation can be carried out in all fixed-bed reactorsknown from the prior art, such as, for example, in the rack oven, in thefixed-bed tubular reactor or tube-bundle reactor, or in the plate heatexchanger reactor. A tube-bundle reactor is preferred. Preferably, theoxidative dehydrogenation is carried out in fixed-bed tubular reactorsor fixed-bed tube-bundle reactors. The reaction tubes are generallyfabricated from steel (just as are the other elements of the tube-bundlereactor). The wall thickness of the reaction tubes is typically 1 to 3mm. The internal diameter thereof is generally (uniformly) 10 to 50 mm,or 15 to 40 mm, frequently 20 to 30 mm. The number of reaction tubesaccommodated in the tube-bundle reactor is generally at least 1000, or3000, or 5000, preferably at least 10 000. Frequently, the number ofreaction tubes accommodated in the tube-bundle reactor is 15 000 to 30000, or up to 40 000, or up to 50 000. The length of the reaction tubesranges in the usual case to a few meters, typically a reaction tubelength is in the range from 1 to 8 m, frequently 2 to 7 m, in many cases2.5 to 6 m.

In addition, the catalyst layer which is installed in the ODH-reactor Acan comprise an individual layer or 2 or more layers. These layers cancomprise a pure catalyst, or be diluted with a material that does notreact with the starting gas or components of the product gas of thereaction. In addition, the catalyst layers can comprise solid materialand/or supported shell catalysts.

The product gas stream 2 leaving the oxidative dehydrogenation, inaddition to butadiene, generally comprises still unreacted 1-butene and2-butene, oxygen and also steam. As minor components, it furthergenerally comprises carbon monoxide, carbon dioxide, inert gases(principally nitrogen), low-boiling hydrocarbons such as methane,ethane, ethene, propane and propene, butane and isobutane, possiblyhydrogen and also possibly oxygen-comprising hydrocarbons, termedoxygenates. Oxygenates can be, for example, formaldehyde, furan, aceticacid, maleic anhydride, formic acid, methacrolein, methacrylic acid,crotonaldehyde, crotonic acid, propionic acid, acrylic acid, methylvinyl ketone, styrene, benzaldehyde, benzoic acid, phthalic anhydride,fluorenone, anthraquinone and butyraldehyde.

The product gas stream 2 at the reactor exit is characterized by atemperature close to the temperature at the end of the catalyst bed. Theproduct gas stream is then brought to a temperature from 150 to 400° C.,preferably 160 to 300° C., particularly preferably 170 to 250° C. It ispossible to insulate the line through which the product gas stream flowsor to use a heat exchanger in order to keep the temperature in thedesired range. This heat-exchange system is of any desired type,provided that with this system the temperature of the product gas can bekept at the desired level. As an example of a heat exchanger, spiralheat exchangers, plate heat exchangers, double-tube heat exchangers,multitube heat exchangers, boiler-spiral heat exchangers, boiler-shellheat exchangers, liquid-liquid contact heat exchangers, air-heatexchangers, direct-contact heat exchangers and also finned-tube heatexchangers may be mentioned. Since, while the temperature of the productgas is being set to the desired temperature, a part of the high-boilingbyproducts that are present in the product gas can precipitate out, theheat-exchanger system should therefore preferably have two or more heatexchangers. If, in this case, two or more of the provided heatexchangers are arranged in parallel, and thus a distributed cooling ofthe product gas obtained is permitted in the heat exchangers, the amountof high-boiling byproducts that are deposited in the heat exchangersdecreases, and thus the operating time thereof can be prolonged. As analternative to the abovementioned method, the two or more provided heatexchangers can be arranged in parallel. The product gas is fed to one ormore, but not all, heat exchangers which, after a certain operatingtime, are detached from other heat exchangers. In this method, thecooling can be continued, a part of the heat of reaction recovered, andin parallel thereto, the high-boiling byproducts deposited in one of theheat exchangers can be removed. As a refrigerant mentioned above, asolvent can be used, provided that it is able to dissolve thehigh-boiling byproducts. Examples are aromatic hydrocarbon solvents suchas, e.g. toluene, xylenes, diethylbenzenes, triethylbenzenes,diisopropylbenzenes and triisopropylbenzenes. Particular preference isgiven to mesitylene. Aqueous solvents can also be used. These can bemade either acidic or else alkaline, such as, for example, an aqueoussolution of sodium hydroxide.

Then, by cooling and compression, a majority of the high-boiling minorcomponents and the water are separated off from the product gas stream2. The cooling proceeds by contacting with a refrigerant. This stage isthen also termed quench. This quench can comprise only one stage or aplurality of stages (for example B, C in FIG. 1). The product gas stream2 is therefore brought directly into contact with the organic coolingmediums 3 b and 9 b, and thereby cooled. As cooling medium, aqueousrefrigerants or organic solvents are suitable, preferably aromatichydrocarbons, particularly preferably toluene, o-xylene, m-xylene,p-xylene or mesitylene, or mixtures thereof. Diethylbenzene,triethylbenzene, diisopropylbenzene and triisopropylbenzene can also beused.

Preference is given to a two-stage quench (comprising the stages B and Cas per FIG. 1), i.e. the stage Ca) comprises two cooling stages Ca1) andCa2), in which the product gas stream 2 is brought into contact with theorganic solvent.

Generally, the product gas 2 has a temperature from 100 to 440° C.,depending on presence and temperature level of a heat exchanger upstreamof the quench B. The product gas is contacted with the cooling medium oforganic solvent in the 1st quench stage B. In this case, the coolingmedium can be introduced via a nozzle in order to achieve as efficientas possible mixing with the product gas. For the same purpose, internalssuch as, for example, further nozzles can be introduced in the quenchstage, through which internals the product gas and the cooling mediumpass together. The refrigerant inlet into the quench is designed in sucha manner that blockage by deposits in the region of the refrigerantinlet is minimized.

Generally, the product gas 2 is cooled in the first quench stage B to 5to 180° C., preferably to 30 to 130° C., and still more preferably to 60to 110° C. The temperature of the refrigerant medium 3 b at the inletcan be generally 25 to 200° C., preferably 40 to 120° C., particularlypreferably 50 to 90° C. The pressure in the first quench stage B is notparticularly restricted, but is generally 0.01 to 4 bar (gauge),preferably 0.1 to 2 bar (gauge) and particularly preferably 0.2 to 1 bar(gauge). If larger amounts of high-boiling byproducts are present in theproduct gas, polymerization of high-boiling byproducts and deposits ofsolids that are caused by high-boiling byproducts in this method sectioncan easily occur. Generally, the quench stage B is designed as a coolingtower. The cooling medium 3 b used in the cooling tower is frequentlyused in circulation. The circuit stream of the cooling medium in litersper hour, based on the mass flow rate of butadiene in grams per hour,can generally be 0.0001 to 5 l/g, preferably 0.001 to 1 l/g, andparticularly preferably 0.002 to 0.2 l/g.

The temperature of the cooling medium 3 in the sump can be generally 27to 210° C., preferably 45 to 130° C., particularly preferably 55 to 95°C. Since the loading of the cooling medium 4 with minor componentsincreases over the course of time, a part of the loaded cooling mediumcan be taken off from the circulation as purge stream 3 a and the amountin circulation can be kept constant by addition of non-loaded coolingmedium 6. The ratio of amount in circulation and amount of additiondepends on the vapor loading of the product gas and the product gastemperature at the end of the first quench stage.

The product gas stream 4 that is also possibly depleted in minorcomponents can then be fed to a second quench stage C. Therein, it canagain be brought into contact with cooling medium 9 b.

Generally, the product gas can be cooled up to the gas exit of thesecond quench stage C to 5 to 100° C., preferably to 15 to 85° C., andstill more preferably to 30 to 70° C. The refrigerant can be fed incounterflow to the product gas. In this case, the temperature of therefrigerant medium 9 b at the refrigerant inlet can be 5 to 100° C.,preferably 15 to 85° C., particularly preferably 30 to 70° C. Thepressure in the second quench stage C is not particularly restricted,but is generally 0.01 to 4 bar (gauge), preferably 0.1 to 2 bar (gauge)and particularly preferably 0.2 to 1 bar (gauge). The second quenchstage is preferably designed as a cooling tower. The cooling medium 9 bused in the cooling tower is frequently used in circulation. The circuitstream of the cooling medium 9 b in liters per hour, based on the massflow rate of butadiene in grams per hour, can be generally 0.0001 to 5l/g, preferably 0.001 to 1 l/g, and particularly preferably 0.002 to 0.2l/g.

Depending on temperature, pressure, refrigerant and water content of theproduct gas 4, condensation of water can occur in the second quenchstage C. In this case, an additional aqueous phase 8 can form, which canadditionally comprise water-soluble minor components. These can then betaken off in the phase separator D. The temperature of the coolingmedium 9 in the sump can be generally 20 to 210° C., preferably 35 to120° C., particularly preferably 45 to 85° C. Since the loading of thecooling medium 9 with minor components increases over the course oftime, a part of the loaded cooing medium can be taken off from thecirculation as purge stream 9 a, and the amount in circulation can bekept constant by addition of non-loaded cooling medium 10.

In order to achieve the best possible contact between product gas andcooling medium, internals can be present in the second quench stage C.Such internals comprise, for example, bubble-cap, centrifugal and/orsieve trays, columns having structured sheet-metal packings having aspecific surface area from 100 to 1000 m²/m³ such as Mellapak® 250 Y,and randomly packed columns.

The solvent circulations of the two quench stages can be either separatefrom one another or connected to one another. For instance, the stream 9a can be fed to the stream 3 b, or replace it. The desired temperatureof the circulation streams can be set via suitable heat exchangers.

In a preferred embodiment of the invention, therefore, the cooling stageCa) is carried out in a two-stage manner, wherein the solvent loadedwith minor components of the second stage Ca2) is conducted into thefirst stage Ca1). The solvent withdrawn from the second stage Ca2)contains less minor components than the solvent withdrawn from the firststage Ca1).

In order to minimize the entrainment of liquid components from thequench into the off-gas line, suitable structural measures such as, forexample, the installation of a demister, can be taken. In addition,high-boiling substances which are not separated off from the product gascan be removed from the product gas by further structural measures suchas, for example, further gas scrubbing stages.

A gas stream 5 is obtained which comprises n-butane, 1-butene,2-butenes, butadiene, possibly oxygen, hydrogen, steam, in small amountsmethane, ethane, ethene, propane and propene, isobutane, carbon oxides,inert gases and parts of the solvent used in the quench. In addition,traces of high-boiling components can remain in this gas stream 5, whichhigh-boiling components have not been separated off quantitatively inthe quench.

Then, the gas stream b from the cooling stage Ca) which is depleted inhigh-boiling minor components, is cooled in step Cb) in at least onecompression stage Cba) and preferably in at least one cooling stage Cbb)by contacting with an organic solvent as cooling agent.

The product gas stream 5 from the solvent quench is compressed in atleast one compression stage E and then further cooled in the coolingapparatus F, wherein at least one condensate stream 14 is formed. A gasstream 12 remains comprising butadiene, 1-butene, 2-butenes, oxygen,steam, possibly low-boiling hydrocarbons such as methane, ethane,ethene, propane and propene, butane and isobutane, possibly carbonoxides and possibly inert gases. In addition, said product gas streamcan further comprise traces of high-boiling components.

The compression and cooling of the gas stream 5 can proceed in asingle-stage or multistage (n-stage) manner. Generally, in total,compression proceeds from a pressure in the range from 1.0 to 4.0 bar(absolute) to a pressure in the range from 3.5 to 20 bar (absolute).After each compression stage, a cooling stage follows in which the gasstream is cooled to a temperature in the range from 15 to 60° C. Thecondensate stream can therefore, in the case of multistage compression,also comprise a plurality of streams. The condensate stream compriseslarge parts of water and the solvent used in the quench. Both streams(aqueous and organic phases) can in addition comprise to a small extentminor components such as low-boilers, C₄ hydrocarbons, oxygenates andcarbon oxides.

In order to cool stream 11 resulting from compression of stream 5 and/orin order to remove further minor components from the stream 11, thecondensed quench solvent can be cooled in a heat exchanger andrecirculated as refrigerant to the apparatus F. Since the loading ofthis cooling medium 13 b with minor components increases over the courseof time, a part of the loaded cooling medium can be taken off from thecirculation (13 a) and the amount of cooling medium in circulation canbe kept constant by addition of non-loaded solvent (15).

The solvent 15 that is added as cooling medium can be an aqueousrefrigerant or an organic solvent. Preference is given to aromatichydrocarbons, particular preference to toluene, o-xylene, m-xylene,p-xylene, diethylbenzene, triethylbenzene, diisopropylbenzene,triisopropylbenzene, mesitylene or mixtures thereof. Particularpreference is given to mesitylene.

The condensate stream 13 a can be recirculated to the circuit stream 3 band/or 9 b of the quench. As a result, the C₄ components absorbed in thecondensate stream 13 a can again be brought into the gas stream and theyield can thereby be increased.

Suitable compressors are, for example, turbo compressors, rotary pistoncompressors and reciprocating piston compressors. The compressors can bedriven for example, by an electric motor, an expander, or a gas or steamturbine. The input pressure into the first compressor stage is 0.5 to 3bar absolute, preferably 1 to 2 bar absolute. Typical compression ratios(exit pressure:entry pressure) per compressor stage are, depending onconstruction type, between 1.5 and 3.0. The cooling of the compressedgas proceeds in refrigerant-flushed heat exchangers or organic quenchstages that can be constructed, for example, as tube-bundle, spiral orplate heat exchangers. Suitable refrigerants can be aqueous or theabovementioned organic solvents. As refrigerants in the heat exchangers,in this case, cooling water or heat-transfer oils or organic solventsare used. In addition, preferably air cooling is used for the use ofblowers.

The gas stream 12 comprising butadiene, n-butenes, oxygen, low-boilinghydrocarbons (methane, ethane, ethene, propane, propene, n-butane,isobutane), possibly steam, possibly carbon oxides and also possiblyinert gases, and possibly traces of minor components, is fed as outputstream to the further treatment.

In a step D), non-condensable and low-boiling gas components comprisingoxygen, low-boiling hydrocarbons (methane, ethane, ethene, propane,propene), carbon oxides and inert gases are separated off in anabsorption column G as gas stream 16 from the process gas stream 12 byabsorption of the C₄ hydrocarbons in a high-boiling absorbent (21 band/or 26) and subsequent desorption of the C₄ hydrocarbons. Preferably,step D), as shown in FIG. 1, comprises the steps Da1), Da2) and Db):

-   Da1) absorption of the C₄ hydrocarbons comprising butadiene and    n-butenes in a high-boiling absorbent (21 b and/or 26), wherein an    absorbent stream loaded with C₄ hydrocarbons and the gas stream 16    are obtained,-   Da2) removal of oxygen from the absorbent stream of step Da1) that    is loaded with C₄ hydrocarbons by stripping with a non-condensable    gas stream 18, wherein an absorbent stream 17 loaded with C₄    hydrocarbons is obtained, and-   Db) desorption of the C₄ hydrocarbons from the loaded absorbent    stream, wherein a C₄- product gas stream 27 is obtained which    substantially comprises C₄ hydrocarbons.

For this purpose, in the absorption stage G, the gas stream 12 isbrought into contact with an absorbent and the C₄ hydrocarbons areabsorbed in the absorbent, wherein an absorbent loaded with C₄hydrocarbons and an off-gas 16 comprising the remaining gas componentsare obtained. In a desorption stage H the C₄ hydrocarbons are liberatedagain from the high-boiling absorbent.

The absorption stage can be carried out in any desired suitableabsorption column known to those skilled in the art. The absorption canproceed by simply passing the product gas stream through the absorbent.However, it can also proceed in columns or in rotary absorbers. In thiscase, cocurrent flow, counterflow or cross flow can be employed.Preferably, the absorption is carried out in counterflow. Suitableabsorption columns are, e.g., tray columns having bubble-cap,centrifugal and/or sieve trays, columns having structured packings, e.g.sheet metal packings having a specific surface area from 100 to 1000m²/m³ such as Mellapak® 250 Y, and randomly packed columns. However,trickling towers and spray towers, graphite block absorbers, surfaceabsorbers such as thick-layer and thin-layer absorbers and also rotarycolumns, disk scrubbers, cross-spray scrubbers and rotary scrubbers alsocome into consideration.

In an embodiment, the gas stream 12 comprising butadiene, n-butenes andthe low-boiling and non-condensable gas components is fed to anabsorption column in the lower region. In the upper region of theabsorption column, the high-boiling absorbent (21 b and/or 26) isapplied.

Inert absorbents used in the absorption stage are generallyhigh-boiling, nonpolar solvents in which the C₄ hydrocarbon mixture thatis to be separated off has a markedly higher solubility than theremaining gas components that are to be separated off. Suitableabsorbents are comparatively nonpolar organic solvents, for examplealiphatic C₈- to C₁₈ alkanes, or aromatic hydrocarbons such as themiddle oil fractions of paraffin distillation, toluene or ethers havingbulky groups, or mixtures of said solvents, wherein a polar solvent suchas 1,2-dimethyl phthalate can be added thereto. Suitable absorbents are,in addition, esters of benzoic acid and phthalic acid havingstraight-chain C₁- to C₈ alkanols, and also what are termed heat carrieroils, such as biphenyl and diphenyl ether, the chlorine derivativesthereof and also triarylalkenes. A suitable absorbent is a mixture ofbiphenyl and diphenyl ether, preferably in the azeotropic composition,for example the commercially available Diphyl®. Frequently, this solventmixture comprises dimethyl phthalate in an amount from 0.1 to 25% byweight.

In a preferred embodiment, the same solvent is used in the absorptionstage Da1) as in the cooling stage Ca).

Preferred absorbents are solvents that have a dissolving power fororganic peroxides of at least 1000 ppm (mg of active oxygen/kg ofsolvent). Preference is given to aromatic hydrocarbons, particularlypreferably toluene, o-xylene, p-xylene and mesitylene, or mixturesthereof. Use can also be made of diethylbenzene, triethylbenzene,diisopropylbenzene and triisopropylbenzene.

At the top of the absorption column G, a stream 16 is taken off thatsubstantially comprises oxygen, low-boiling hydrocarbons (methane,ethane, ethene, propane, propene), possibly C₄ hydrocarbons (butane,butenes, butadiene), possibly inert gases, possibly carbon oxides andpossibly also steam. This material stream can in part be fed to the ODHreactor. By this means, for example the entry stream up to the ODHreactor may be adjusted to the desired Ca hydrocarbon content.

At the sump of the absorption column, by purging with a gas 18, residuesof oxygen dissolved in the absorbent may be discharged. The remainingoxygen fraction is to be so small that the stream 27 leaving thedesorption column and comprising butane, butene and also butadiene,comprises only a maximum of 100 ppm of oxygen.

The oxygen can be stripped out in step Db) in any desired suitablecolumn known to those skilled in the art. The stripping can be performedby simple passage of non-condensable gases, preferably non-absorbablegases, or only slightly absorbable gases in the absorbent stream 21 band/or 26 such as methane, through the loaded absorption solution.Co-stripped C₄ hydrocarbons are scrubbed back into the absorptionsolution in the upper part of the column G by passing the gas streamback into this absorption column. This can be performed either bypipework of the stripper column, and also by direct assembly of thestripper column beneath the absorber column. Since the pressure in thestripping column part and absorption column part is the same, this canproceed by direct coupling. Suitable stripping columns are, e.g., traycolumns having bubble-cap, centrifugal and/or sieve trays, columnshaving structured packings, e.g. sheet-metal packings having a specificsurface area from 100 to 1000 m²/m³ such as Mellapak® 250 Y, andrandomly packed columns. However, trickle towers and spray towers andalso rotary columns, disk scrubbers, cross-spray scrubbers and rotaryscrubbers also come into consideration. Suitable gases are, for example,nitrogen or methane.

Stream 17 can optionally be cooled or heated and enters into thedesorption column H as stream 19. The entry point is generally 0 to 10theoretical separation plates, preferably 2 to 8 theoretical separationplates, particularly preferably 3 to 5 theoretical separation platesbeneath the column head.

The absorbent regenerated in the desorption stage is withdrawn from thedesorption column H as stream 20, together with the condensed water.This two-phase mixture can be cooled in a heat exchanger and, as stream21, be separated in a decanter I into an aqueous stream 21 a and anabsorbent stream 21 b. The absorbent stream 21 b is fed back to theabsorber column G, while the aqueous stream 21 a is evaporated in anevaporator and stream 23 generated thereby. Additionally or as analternative, additional water (stream 24) can further be evaporated inthe evaporator. In addition, also, only a part of the stream 21 a can beevaporated, and the non-evaporated part be withdrawn as stream 22 andfed, for example, to wastewater treatment.

Low-boilers such as, for example, ethane or propane, and alsohigh-boiling components such as benzaldehyde, maleic anhydride andphthalic anhydride situated in the process gas stream can accumulate inthe absorbent circulation stream. In order to restrict the accumulation,a purge stream 25 can be taken off.

The C₄ product gas stream 27 substantially comprising n-butane,n-butenes and butadiene generally comprises 20 to 80% by volume ofbutadiene, 0 to 80% by volume of n-butane, 0 to 10% by volume of1-butene and 0 to 50% by volume of 2-butenes, wherein the total amountis 100% by volume. In addition, small amounts of isobutane may bepresent.

A part of the condensed, principally C₄ hydrocarbon-comprising overheaddischarge of the desorption column is recirculated as stream 30 to thecolumn head in order to increase the separation efficiency of thecolumn.

The desorption stage H can be carried out in any desired suitabledesorption column known to those skilled in the art. The desorption canproceed by lowering the pressure and/or heating the desorption stage.The desorption stage can be heated by supplying a hot medium—as, forexample, steam—or by internal vapor which is generated, e.g., by partialevaporation of the absorption solution in the sump of the desorptioncolumn.

Suitable desorption columns are, e.g., tray columns having bubble-cap,centrifugal and/or sieve trays, columns having structured packings, e.g.sheet-metal packings having a specific surface area from 100 to 1000m²/m³, such as Mellapak® 250 Y, and randomly packed columns. Accordingto the invention, as shown in FIG. 1, a methacrolein-comprising sidetakeoff stream 31 can be withdrawn from the desorption column H in orderto prevent an increase in the concentration of acrolein in the absorbentcircuit stream. The side takeoff stream 31 can be either liquid or elsegaseous, preferably it is gaseous.

Preferably, the desorption column H has 5 to 30, particularly preferably10 to 20, theoretical plates. The side takeoff stream 31 is preferablywithdrawn here in the lower third of the desorption column. The liquidside takeoff stream 31 generally comprises 0.1 to 2% by weightmethacrolein. In addition, it comprises 5 to 15% by weight of water, 0to 3% by weight of C₄ hydrocarbons and 70 to 90% by weight of theabsorbent.

The gaseous side takeoff stream 31 generally comprises 1 to 10% byweight of methacrolein. In addition, it comprises 30 to 60% by weight ofwater, 0 to 6% by weight of C₄ hydrocarbons and 30 to 60% by weight ofthe absorbent.

The gaseous C₄ hydrocarbon-comprising stream 29 is recirculated to thecompressor E.

Preferably, the polymerization inhibitor is added to the top condenserof the desorption column H together with the stream 27 a. Saidpolymerization inhibitor can be added in solid form, as solution oremulsion. Preferably, it is added as a solution. Particularlypreferably, it is added as an aqueous solution. The solution cancomprise one or more different stabilizers. Preferred polymerizationinhibitors (stabilizers) are selected from the group of unsubstitutedand substituted catechols and hydroquinones.

Generally, the polymerization inhibitor is added in an amount such thatthe concentration thereof in the liquid condensate obtained at the topcondenser is from 10 to 500 ppm, preferably 30 to 100 ppm.

Therefore, the concentration of the polymerization inhibitor in stream28 and in the reflux 30 is 10 to 500 ppm, preferably 30 to 100 ppm.

Preference is given to a mixture of at least one stabilizer from theclass of catechols and at least one stabilizer from the class ofhydroquinones. Particular preference is given to a mixture of tert-butylcatechol and 4-methoxyphenol.

The liquid stream 28 leaving the top condenser and comprising the C₄hydrocarbons is then evaporated in stage N and the resulting stream 28 ais separated by extractive distillation in step E) using a solventselective for butadiene into a material stream 35 comprising butadieneand the selective solvent, and a material stream 36 comprising butanesand n-butenes.

In an embodiment stream 28 can be scrubbed previously in a liquid-liquidscrubbing with polyalcohols such as ethylene glycol and glycerol ormethanol, and the furan present therein can be inpart separated off. Ina further embodiment, the stream 28 a can be freed in advance from otherminor components such as aldehydes in a gas-liquid scrubbing with water.

The extractive distillation can be carried out, for example as describedin “Erdöl and Kohle-Erdgas-Petrochemie” [Petroleum and coal—naturalgas—petrochemistry], volume 34 (8), pages 343 to 346, or “UllmannsEnzyklopädie der Technischen Chemie” [Ullmann's encyclopedia ofindustrial chemistry], volume 9, 4th edition 1975, pages 1 to 18. Forthis purpose, the C₄ product gas stream is contacted with an extractionmedium, preferably an N-methylpyrrolidone (NMP)/water mixture, in anextraction zone. The extraction zone is generally designed in the formof a scrubbing column which comprises trays, random packings orstructured packings as internals. Said scrubbing column generally has 30to 70 theoretical separation plates, in order that a sufficiently goodseparation efficiency is achieved. Preferably, the scrubbing column hasa backwash zone in the column head. This backwash zone serves forrecovery of the extraction medium present in the gas phase using aliquid hydrocarbon reflux, for which purpose the overhead fraction iscondensed in advance. The mass ratio of extraction medium to C₄ productgas stream in the feed to the extraction zone is generally 10:1 to 20:1.The extractive distillation is preferably operated at a sump temperaturein the range from 100 to 250° C., in particular at a temperature in therange from 110 to 210° C., a head temperature in the range from 10 to100° C., in particular in the range from 20 to 70° C., and a pressure inthe range from 1 to 15 bar, in particular in the range from 3 to 8 bar.The extractive distillation column preferably has 5 to 70 theoreticalseparation plates.

Suitable extraction media are butyrolactone, nitriles such asacetonitrile, propionitrile, methoxypropionitrile, ketones such asacetone, furfural, N-alkyl substituted lower aliphatic acid amides suchas dimethylformamide, diethylformamide, dimethylacetamide,diethylacetamide, N-formylmorpholine, N-alkyl substituted cyclic acidamides (lactams) such as N-alkylpyrrolidones, in particularN-methylpyrrolidone (NMP). Generally, alkyl-substituted lower aliphaticacid amides or N-alkyl substituted cyclic acid amides are used.Dimethylformamide, acetonitrile, furfural and, in particular, NMP areparticularly advantageous.

However, mixtures of these extraction media with one another, e.g. ofNMP and acetonitrile, mixtures of these extraction media withco-solvents and/or tert-butyl ether, e.g. methyl tert-butyl ether, ethyltert-butyl ether, propyl tert-butyl ether, n-butyl or isobutyltert-butyl ether can also be used. NMP is particularly suitable,preferably in an aqueous solution, preferably with 0 to 20% by weight ofwater, particularly preferably with 7 to 10% by weight of water, inparticular with 8.3% by weight of water.

The overhead product stream 36 of the extractive distillation column Jsubstantially comprises butane and butenes and in small amountsbutadiene, and is taken off in the gaseous or liquid state. Generally,the stream that substantially comprises n-butane and 2-butene comprisesup to 100% by volume of n-butane, 0 to 50% by volume of 2-butene and 0to 3% by volume further components such as isobutane, isobutene,propane, propene and C₅ ⁺ hydrocarbons.

The stream substantially comprising n-butane and 2-butene and possiblymethane can be fed in whole or in part or else not into the C₄ feed ofthe ODH reactor. Since the butene isomers of this reflux streamsubstantially comprise 2-butenes, and 2-butenes are generallyoxidatively dehydrogenated more slowly to butadiene than is 1-butene,this reflux stream, before it is fed to the ODH reactor, can becatalytically isomerized. As a result, the isomeric distribution can beadjusted in accordance with the isomeric distribution present inthermodynamic equilibrium. In addition, the stream can be fed to afurther workup, in order to separate butanes and butenes from oneanother and to recirculate the butenes in whole or in part to the oxydehydrogenation. The stream can also pass into the maleic anhydrideproduction.

In a step F), the butadiene and the material stream comprising selectivesolvents are separated by distillation into a material streamsubstantially comprising the selective solvent and abutadiene-comprising material stream.

The material stream 35 obtained at the sump of the extractivedistillation column J generally comprises the extraction medium, water,butadiene, and, in small fractions, butenes and butane, and is fed to adistillation column K. Butadiene can be obtained overhead or as a sidetakeoff. At the sump of the distillation column, an extraction mediumand possibly water-comprising material stream 37 occurs, wherein thecomposition of the extraction medium and water-comprising materialstream corresponds to the composition as is added to the extraction. Theextraction medium and the water-comprising material stream is preferablyreturned to the extractive distillation.

If the butadiene is obtained by a side takeoff, the extraction solutionthus taken off is transferred to a desorption zone, wherein thebutadiene is once again desorbed from the extraction solution andbackwashed. The desorption zone can be designed, for example, in theform of a scrubbing column that has 2 to 30, preferably 5 to 20,theoretical plates, and optionally a backwash zone having, for example,4 theoretical plates. This backwash zone serves for recovery of theextraction medium present in the gas phase using a liquid hydrocarbonreflux, for which purpose the overhead fraction is condensed in advance.As internals, structured packings, trays or random packings areprovided. The distillation is preferably carried out at a sumptemperature in the range from 100 to 300° C., in particular in the rangefrom 150 to 200° C., and an overhead temperature in the range from 0 to70° C., in particular in the range from 10 to 50° C. The pressure in thedistillation column in this case is preferably in the range from 1 to 10bar. Generally, in the desorption zone, a pressure reduced in comparisonwith the extraction zone prevail and/or an elevated temperature.

The valuable product stream 38 obtained at the column head generallycomprises 90 to 100% by volume of butadiene, 0 to 10% by volume of2-butene and 0 to 10% by volume of n-butane and isobutane. For furtherpurification of the butadiene, a further distillation according to theprior art can be carried out.

The claimed technical solution was developed via thermodynamicequilibrium study simulations, and examined in a pilot plant.

This pilot plant comprises the salt bath reactor, the organic quench,the compressor unit and also the C₄ absorption/desorption unit. Thescale of the pilot plant was selected in such a manner that up-scalingto a large scale plant is possible. The internals of the columns and therefluxes were accordingly selected so as to be representative. The pilotplant can produce between 500 and 1500 grams of butadiene per hour.

COMPARATIVE EXAMPLE

On operation of the plant, polymeric deposits from amethacrolein-butadiene copolymer were found in the topmost columnsection of desorber column H after 10 days of operation.

In addition, polymeric deposits were also found in the C₄ evaporator (Nin FIG. 1). The polymer formation at this point was so dominant that thecoiled tube evaporator of the plant became blocked.

Example 1

The polymers found have been with high probability formed via afree-radical mechanism. As a countermeasure, a free-radical trap wasadded to the condenser at the top of the desorber column. By selectionof this point of addition, both the condenser and the downstreamcomponents were protected from polymers by the free-radical trap. Saidfree-radical trap distributed itself via the C₄ reflux stream 30 intothe desorber column and protects the upper column sections which are notprotected by an inhibition of the absorbent conducted in circulation. Inaddition, it also flows together with the stream 28 into the C₄evaporator and also there prevents the formation of polymers. Inexperiments in the miniplant system with the fluxes tabulatedhereinafter, it has been found that fault-free operation was achieved bythese measures.

The composition of the individual material streams is shown in table 1.

TABLE 1 Stream: 12 16 17 18 19 20 21 21a 21b Temperature ° C. 54.1 32.452.9 35.0 60.0 148.0 30.0 30.0 30.0 Pressure bar 10.0 10.0 10.0 10.0 5.55.5 10.3 10.2 10.1 Mass flow rate kg/h 9.5 8.3 22.0 0.3 22.0 22.8 22.82.3 20.1 BUTANE % by weight 3.34 0.53 1.26 0.00 1.26 0.01 0.01 0.00 0.01I-BUTANE 0.74 0.10 0.28 0.00 0.28 0.00 0.00 0.00 0.00 1-BUTENE 0.02 0.000.01 0.00 0.01 0.00 0.00 0.00 0.00 C-2-BUTENE 0.58 0.01 0.25 0.00 0.250.00 0.00 0.00 0.01 T-2-BUTENE 1.36 0.02 0.59 0.00 0.59 0.01 0.01 0.000.01 1.3-BUTADIENE 9.65 0.40 4.10 0.00 4.10 0.05 0.05 0.00 0.06 Water0.87 0.30 0.38 0.00 0.38 10.52 10.52 99.42 0.04 ACROLEIN 0.17 0.13 0.680.00 0.68 0.67 0.67 0.23 0.72 ACETALDEHYDE 0.13 0.01 0.07 0.00 0.7 0.020.02 0.03 0.02 Methacrolein 0.28 0.25 1.01 0.00 1.01 0.98 0.98 0.18 1.08Mesitylene 0.54 0.21 90.97 0.00 90.97 87.56 87.56 0.01 97.89 Stabilizer0.0000 0.0000 0.0096 0.0000 0.0096 0.0165 0.0165 0.0654 0.0104 CO₂ 0.941.07 0.01 0.00 0.01 0.00 0.00 0.00 0.00 CO 0.19 0.22 0.00 0.00 0.00 0.000.00 0.00 0.00 N₂ 74.78 89.56 0.15 100.00 0.15 0.00 0.00 0.00 0.00 O₂6.16 7.11 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Others 0.26 0.08 0.23 0.000.23 0.15 0.15 0.07 0.16 Stream: 22 23 24 25 26 27 27a 28 29 30Temperature ° C. 30 157 35 30 35 50 30 17 17 17 Pressure bar 10.2 5.616510.2 10.1 10 5.5 5.4 5.3 5.4 5.3 Mass flow rate kg/h 0.15 2.33 0.07 0.300.24 3.39 0.001 1.42 0.08 1.90 BUTANE % by weight 0.00 0.00 0.00 0.010.00 18.68 0.00 18.86 10.46 18.86 I-BUTANE 0.00 0.00 0.00 0.00 0.00 4.200.00 4.21 3.48 4.21 1-BUTENE 0.00 0.00 0.00 0.00 0.00 0.12 0.00 0.120.07 0.12 C-2-BUTENE 0.00 0.00 0.00 0.01 0.00 3.69 0.00 3.74 1.61 3.74T-2-BUTENE 0.00 0.00 0.00 0.01 0.00 8.69 0.00 8.79 4.10 8.791.3-BUTADIENE 0.00 0.00 0.00 0.06 0.00 60.29 0.00 60.83 35.80 60.83Water 99.42 99.43 100.00 0.04 0.00 0.74 10.00 0.75 0.16 0.75 ACROLEIN0.23 0.23 0.00 0.72 0.00 0.19 0.00 0.20 0.01 0.20 ACETALDEHYDE 0.03 0.030.00 0.02 0.00 0.80 0.00 0.80 0.71 0.80 Methacrolein 0.18 0.18 0.00 1.080.00 0.20 0.00 0.21 0.01 0.21 Mesitylene 0.01 0.01 0.00 97.89 100.000.01 0.00 0.01 0.00 0.01 Stabilizer 0.0654 0.0486 0.0000 0.0104 0.00000.0000 90.0000 0.0271 0.0000 0.0271 CO₂ 0.00 0.00 0.00 0.00 0.00 0.060.00 0.04 1.03 0.04 CO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00N₂ 0.00 0.00 0.00 0.00 0.00 1.03 0.00 0.08 42.19 0.08 O₂ 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.01 0.00 others 0.07 0.07 0.00 0.16 0.00 1.300.00 1.32 0.35 1.32

The invention claimed is:
 1. A method for producing butadiene fromn-butenes having the steps: A) providing a feed gas stream a comprisingn-butenes; B) feeding the feed gas stream a comprising n-butenes and anoxygen-comprising gas into at least one oxidative dehydrogenation zoneand oxidatively dehydrogenating n-butenes to butadiene, wherein aproduct gas stream b comprising butadiene, unreacted n-butenes, steam,oxygen, low-boiling hydrocarbons, high-boiling minor components,optionally carbon oxides and optionally inert gases is obtained; Ca)cooling the product gas stream b by contacting it with a refrigerant andcondensing at least a part of the high-boiling minor components; Cb)compressing the remaining product gas stream b in at least onecompression stage, wherein at least one aqueous condensate stream c1 anda gas stream c2 comprising butadiene, n-butenes, steam, oxygen,low-boiling hydrocarbons, optionally carbon oxides and optionally inertgases are obtained; Da) separating off non-condensable and low-boilinggas components comprising oxygen, low-boiling hydrocarbons, optionallycarbon oxides and optionally inert gases as a gas stream d2 from the gasstream c2 by absorbing C₄ hydrocarbons comprising butadiene andn-butenes in an absorbent, wherein an absorbent stream loaded with C₄hydrocarbons and the gas stream d2 are obtained, and Db) subsequentlydesorbing the C₄ hydrocarbons from the loaded absorbent stream in adesorption column, wherein a C₄ product gas stream dl is obtained,wherein the desorption column comprises a top condenser located at ahead of the column, and wherein a polymerization inhibitor is added instep Db) at the top condenser of the desorption column.
 2. The methodaccording to claim 1 further comprising: E) separating the C₄ productgas stream d1 by extractive distillation using a selective solvent forbutadiene into a material stream e1 comprising butadiene and theselective solvent, and a material stream e2 comprising n-butenes; F)distilling the material stream e1 comprising butadiene and the selectivesolvent into a material stream gl substantially comprising the selectivesolvent, and a material stream g2 comprising butadiene.
 3. The methodaccording to claim 1, wherein the polymerization inhibitor is added inamounts such that a concentration of the polymerization inhibitor in aliquid condensate stream obtained at the top condenser is from 10 to1500 ppm.
 4. The method according to claim 3, wherein the polymerizationinhibitor is a mixture of tert-butyl catechol and 4-methoxyphenol. 5.The method according to claim 1, wherein the polymerization inhibitor isselected from the group consisting of unsubstituted or substitutedcatechols and hydroquinones.
 6. The method according to claim 1, whereinthe gas stream d2 that is separated off in step Da) is at least in partrecirculated in step B).
 7. The method according claim 1, wherein thestep Da) comprises the steps Da1) and Da2): Da1) absorbing the C₄hydrocarbons comprising butadiene and n-butenes in a high-boilingabsorbent, wherein the absorbent stream loaded with C₄ hydrocarbons andthe gas stream d2 are obtained, and Da2) removing oxygen from theabsorbent stream of step Da1) that is loaded with C₄ hydrocarbons bystripping with a non-condensable gas stream; and wherein the C₄ productgas stream d1 obtained in step Db) comprises less than 100 ppm ofoxygen.
 8. The method according to claim 1, wherein the absorbent usedin step Da) is an aromatic hydrocarbon solvent.